1. Field of the Invention
The present invention relates to a focus detection device used in single lens reflex cameras. More particularly, this invention relates to the positioning of a linear array of photosensitive elements used to detect the focus state of a lens.
2. Description of Related Art
An image separating-type focus detection device is shown schematically in FIG. 5. The basic structure includes a photographic lens 10, a field mask 11, a condenser lens 12, a pair of re-imaging lenses 13a and 13b, a pair of arrays of photosensitive elements 14a and 14b attached to a single substrate 14, and an aperture mask 15. Light from the subject that passes through different regions on the plane of the exit pupil of photographic lens 10 passes through aperture mask 15 and is formed into an image on the arrays of photosensitive elements 14a and 14b by the pair of re-imaging lenses 13a and 13b. The focus state of photographic lens 10 can be detected from the distance between the two images that are formed.
The arrays of photosensitive elements 14a and 14b consist of multiple photosensitive elements 1--1 through 1-k (k referring to a number of units) arranged one dimensionally, as shown in FIG. 6. Electric charge is produced in the photosensitive elements 1--1 through 1-k in accordance with the intensity of the optical image. This electric charge moves within the photosensitive elements 1--1 through 1-k and is accumulated in accumulators 2-1 through 2-k. The accumulated charge is transferred to a transfer component 4 by opening a gate 3 and is used as information for computing the focus state. Hereafter, each of the photosensitive elements 1--1 through 1-k will be generally referred to as photosensitive element 1-i, and each of the accumulators 2-1 through 2-k will be generally referred to as accumulator 2-i.
Photosensitive elements 1-i that comprise the arrays of photosensitive elements 14a and 14b shown in FIG. 7 are formed as rectangles. When the direction in which they are aligned corresponds with the direction corresponding to the horizontal direction on the photographic screen (the left-right direction in FIG. 7), the following problems result. Numerous edges in the form of vertical lines that exist in the natural subject field are formed on the arrays of photosensitive light elements 14a and 14b as a narrow optical image "a" parallel to the boundary lines D of the photosensitive elements 1-i. If this optical image "a" is displaced in the horizontal direction in the image field by an amount smaller than the element width S to form the optical image "a'" the output of the light elements 1-i does not change. It is therefore impossible to detect this displacement in position.
In order to solve this problem, in Japanese Laid-Open Application No. 55-29731, the photosensitive elements 1-i are arranged so that the boundary lines D between the photosensitive elements 1-i are inclined by a small angle from the direction orthogonal to the direction in which the photosensitive elements 1-i are aligned. With this example, an optical image "a" corresponding to a vertical line in the image field will fall across more than one of the photosensitive elements 1-i. Accordingly, even if the optical image is displaced by an amount smaller than the element width S to form optical image "a'" the output of photosensitive elements 1-i will change, making it possible to detect this displacement in position. However, if the amount of inclination L of photosensitive elements 1-i is too large, the optical image "a" will fall across many of the photosensitive elements 1-i, causing the precision of focus detection to drop. Conversely, if the amount of inclination is too small, the effects of the inclination will be difficult to determine. Consequently, a range of inclination L of 0.5S.ltoreq.L.ltoreq.1.5S is considered suitable, taking into consideration that the optical image "a" is generally a certain width.
Another attempt at solving the above problems is shown in U.S. Pat. No. 4,218,623 to Utagawa, in which the photosensitive elements are positioned at an angle.
FIG. 8 shows an assembly similar to the Japanese publication and U.S. patent described above. In the example shown in FIG. 8, it is desirable to keep the range of focus detection in the image field the same as in the example in FIG. 7. Thus, when the height of the array in the direction orthogonal to the direction in which the photosensitive elements 1-i are aligned is set at the same value m as in FIG. 7, the length n of photosensitive elements 1-i in the lengthwise direction becomes larger than the height m. Consequently, as seen in FIG. 9, when an optical image "b" corresponding to a vertical line in the image field moves from a position where it barely touches the top edge of a certain one of photosensitive elements 1-i to a position completely outside of this photosensitive element into a neighboring element forming optical image "b'", the time required for movement of charge to accumulator 2i to communicate this change within photosensitive element 1-i takes longer. This is because of the extended length of photosensitive elements 1-i. Until the charge reaches accumulator 2i, the change from optical image "b" to optical image "b'" is not produced as a signal, and the optical image "b" becomes an afterimage that has a negative effect on focus detection. In this manner, the afterimage time becomes longer because of the extended length of photosensitive elements 1-i. This creates a problem in the responsiveness of focus detection to changes in the optical image caused by movement of the subject or adjustment in the photographic lens focus.